1. Field of the Invention
The present invention relates to a zoom lens which is suitably used in a video camera, a digital still camera, a personal digital assistance (PDA), or the like, and an imaging apparatus.
2. Description of Related Art
In recent years, the reduction in the size of an imaging device, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) raises reduction in the size of an imaging apparatus, such as a digital still camera or the like, as a whole. In order to reduce the size of the imaging apparatus as a whole, a lens system should be reduced in size as a whole (the full length and outer diameter of the lens should be reduced).
JP-A-2006-235062, JP-A-2005-215385, Japanese Patent No. 3706783, and Japanese Patent No. 3392881 describe a zoom lens of a four-group configuration including four lens groups having positive, negative, positive, and positive refractive power provided in order from the object side. The zoom lens described in these documents is configured such that the aperture diaphragm is moved independently or together with some of the lens groups during zooming.
The zoom lens described in JP-A-2006-235062 is configured such that the aperture diaphragm is moved independently from adjacent lens groups during zooming. In this case, the travel distance of the aperture diaphragm with respect to the image plane during zooming is comparatively large, so a moving mechanism of an aperture diaphragm is extended, making it difficult to reduce the size of the zoom lens. Further, the focal length of the first lens group is comparatively long, so the full length of the lens is extended, making it difficult to reduce the size of the zoom lens.
The zoom lens described in Japanese Patent No. 3706783 is configured such that the aperture diaphragm is moved together with the third lens group during zooming. In this case, the focal length of the third lens group is the same as that of the fourth lens group, so the displacement of the third lens group during zooming is large, making it difficult to reduce the size of the zoom lens. Further, the focal length of the first lens group is comparatively long, so the full length of the lens is extended, making it difficult to reduce the size of the zoom lens.
The zoom lens described in JP-A-2005-215385 and Japanese Patent No. 3392881 is configured such that the aperture diaphragm is moved to the image side at the telephoto end further than at the wide-angle end during zooming. In this case, the distance between the first lens group and the aperture diaphragm increases during telephoto imaging, and the outer diameter of each lens of the first lens group is expanded, making it difficult to reduce the size of the zoom lens.